1. Field of the Invention
This invention relates to a semiconductor light emitting diode, more particularly to a semiconductor light emitting diode having an (Al.sub.x Ga.sub.1-x).sub.y In.sub.1-y P double-hetero structure which emits yellow to orange light with high emission efficiency.
2. Description of the Prior Art
Light emitting diodes (LEDs) that utilize an epitaxial growth layer formed on a compound semiconductor substrate are characterized by low power consumption, long service life, high emission efficiency, high reliability and other merits, and are therefore widely used as light sources in various types of display devices. Among the various III-V group compound semiconductors, (Al.sub.x Ga.sub.1-x).sub.y In.sub.1-y P mixed crystal epitaxially grown on a GaAs single crystal substrate and lattice-matched therewith is of the direct transition type and has maximum band gap energy. Since it is therefore capable of bright emission over the green to red region, its range of utilization is expanding rapidly.
FIG. 12 shows an example of a prior art LED having an (Al.sub.x Ga.sub.1-x).sub.y In.sub.1-y P double-hetero (DH) structure. In this figure, reference numeral 1 designates a GaAs substrate, 2 a buffer layer, 5-7 the (Al.sub.x Ga.sub.1-x).sub.y In.sub.1-y P DH structure section, 9 a contact layer, and 10 and 11 electrodes. A number of improvements have been proposed regarding this type of (Al.sub.x Ga.sub.1-x).sub.y In.sub.1-y P LED, including the provision of an AlGaAs current diffusion layer of a thickness not exceeding 2 .mu.m under the contact layer 9 for diffusing the current injected from the electrode 10 so as to broaden the light emitting range and the provision of a reflecting layer between the DH structure and the substrate for effectively emitting light directed toward the substrate. (U.S. Pat. No. 5,153,889.)
Increasing outdoor use of LEDs has led to a demand for brighter devices and a particularly strong need has arisen for bright LEDs emitting light in the 590-650 nm wavelength band. Although LEDs provided with a so-called window layer have been developed in response to this need, even these are not sufficiently bright. Further, since the Al content of the GaAlAs constituting the current diffusion layer and window layer is made large (0.7 to 0.8 based on the total of all elements as unity; hereinafter the same) so as to make these layers transparent to the emitted light, the LED undergoes progressive deterioration during use owing to the strong tendency for the Al to be oxidized. When an attempt is made to obtain an epitaxial growth layer with high Al concentration, moreover, the mismatching of the crystal lattices becomes large and the resulting lattice defects make it impossible to obtain high emission efficiency.
The present invention was accomplished for overcoming these shortcomings of the prior art and has as its object to provide a semiconductor LED which exhibits high emission efficiency in the 550-650 nm wavelength region and does not deteriorate over long periods of use.